1. Field of the Invention
The invention relates to a V-groove grating mirror preferred for use with an external resonator type wavelength variable light source used in coherent light communication and a measurement technical fields and an external resonator type wavelength variable light source using the V-groove grating mirror.
2. Description of the Related Art
Available as an external resonator type wavelength variable light source is a light source of a placement technique called Rittman type wherein emission light from an end face of a side of a semiconductor laser where a antireflection coating is applied is converted into collimated light through a lens, a wavelength of the collimated light is selected through a diffraction grating for diffraction, and the diffracted light is reflected by a mirror and is again diffracted through the diffraction grating, then is fed back into the semiconductor laser through the lens (for example, refer to JP-A-5-267768, U.S. Pat. No. 5,319,668).
In the light source of the Rittman type placement technique as described above, if a general plane mirror 10 is placed at a normal position and light is fed back as shown in FIG. 7A, no problem arises; however, if an optical axis is inclined with respect to the plane mirror 10 or the plane mirror 10 is inclined with respect to the optical axis as shown in FIG. 7B, the reflected light goes in a different direction from the original optical path.
Therefore, hitherto, high accuracy has been required for placement of a mirror.
Likewise, a light source of Rittman type placement technique adopting a mirror whose surface is shaped like V at 90 degrees is known according to U.S. Pat. No. 5,594,744.
The mirror surface is shaped like V at 90 degrees, whereby if the V-shaped mirror 9 or an optical axis is inclined, light can be reflected in parallel with the original optical path because of the V-shaped surface of the mirror 9, as shown in FIG. 6A.
However, with the V-shaped mirror 9, as the incidence position of collimated light shifts from the center of the V-shaped surface, collimated (parallel) move distance of reflected light grows as shown in FIG. 6B and when the reflected light is fed back into a semiconductor laser (hereinafter, abbreviated as LD) 5 through a lens 6, the joint loss is increased as shown in FIG. 5; this is a problem.
It is an object of the invention to provide a V-groove grating mirror capable of providing reflected light as collimated light if the mirror or an optical axis is inclined and suppressing the collimated (parallel) move distance of reflected light to a small distance as much as possible.
It is also an object of the invention to provide an external resonator type wavelength variable light source using the V-groove grating mirror.
To the above ends, the invention according to first aspect is characterized by a V-groove grating mirror comprising reflecting faces with an arrangement like a grating, of a plurality of V grooves 2 each shaped like V at right angles in cross section on a base material 1, for example, as shown in FIG. 1.
In this way, according to the first aspect of the invention, the V-groove grating mirror comprises reflecting faces with an arrangement like a grating, of a plurality of V grooves each shaped like V at right angles in cross section, so that if the V-groove grating mirror or an optical axis is inclined, reflected light in parallel with the incidence light can be provided by means of the V-groove grating of the reflecting faces.
Moreover, a plurality of the V grooves are arranged like a grating at small pitches, so that the collimated (parallel) move distance of the reflected light can be suppressed to a small distance as compared with a V-shaped mirror in a related art.
The invention according to second aspect is characterized by the V-groove grating mirror according to the first aspect of the invention further comprising a reflection coating 3 formed on the surface of the V groove 2, for example, as shown in FIG. 1.
The invention according to third aspect is characterized by the fact that in the V-groove grating mirror according to the first aspect of the invention, the base material 1 is made of a light transmission material and the face opposed to the reflecting faces with the V grooves 2 arranged like a grating is an incidence plane 4, for example, as shown in FIG. 2.
According to the third aspect of the invention, the V-groove grating mirror is made of a light transmission material and the face opposed to the reflecting faces with the V grooves according to the first aspect of the invention arranged like a grating is an incidence plane, so that incidence light passing through an inside of the V-groove mirror is reflected by the V-groove grating and is emitted to an outside of the V-groove mirror.
The invention according to fourth aspect is characterized by the fact that in the V-groove grating mirror according to the first aspect of the invention, a pitch between one groove and the adjacent groove is in a range of from (d/1000) to d where d is the beam diameter of the incident light
The invention according to fifth aspect is characterized by an external resonator type wavelength variable light source comprising:
a LD 5 formed on one end face with a antireflection coating 5a; 
a lens 6 for converting emitted light from the end face of the LD 5 formed with the antireflection coating 5a into collimated light;
a diffraction grating 7 for selecting a wavelength of the collimated light provided through the lens 6 and diffracting the light; and
a mirror for reflecting the light diffracted through the diffraction grating 7 and feeding back the light again diffracted through the diffraction grating 7 into the LD 5 through the lens 6,
wherein a V-groove grating mirror according to the first, second, third, or fourth aspect of the invention is used as the mirror, for example, as shown in FIG. 3.
According to the fifth aspect of the invention, the external resonator type wavelength variable light source uses the v-groove grating according to the first, second, third or fourth aspect of the invention as the mirror for reflecting the light diffracted through the diffraction grating and feeding back the light again diffracted through the diffraction grating into the LD through the lens, so that optical axis adjustment and incidence position adjustment of the mirror become unnecessary, a self-correction of an optical axis shift accompanying wavelength varying can be made, and alignment becomes easy to make as compared with use of the V-shaped mirror in the related art.
The invention according to sixth aspect is characterized by the fact that in the external resonator type wavelength variable light source according to the fifth aspect of the invention, the grating of the V-groove grating mirror 8 is placed in a direction orthogonal to a grating direction of the diffraction grating 6, for example, as shown in FIG. 3.
According to the sixth aspect of the invention, in the external resonator type wavelength variable light source according to the fifth aspect of the invention, the grating of the V-groove grating mirror is placed in the direction orthogonal to the grating direction of the diffraction grating, so that the diffracted light whose wavelength is selected through the diffraction grating can be reliably fed back into the LD by means of the V-groove grating mirror.